1. Field of the Invention
This invention is in the field of current transformers and, more particularly, spit core current transformers.
2. State of the Prior Art
Current transformers are common devices used for measuring AC current flow in electric wires or bus bars, typically, but not exclusively, in higher power installations and equipment. High power as used in this description is not intended to be limiting, but generally refers to electric power with voltages above twenty volts, as opposed to low voltage electronic circuits that operate with less than twenty volts. Essentially, a current transformer outputs a small current that is proportional to a larger current flowing in a high power electric wire or bus bar, and the use of a burden resistor on the output can provide a low voltage signal that is proportional to the current flowing in the high power electric wire or bus bar. Such small current or low voltage output signals from the current transformer can be used in a variety of instrumentation and control applications, including, for example, measuring and/or metering the amount of electric current that is generated or flowing to a load, or measuring and/or metering the amount of power that is used by a load.
A typical current transformer comprises a magnetic core, a primary winding (which may be the high power wire or bus bar), and a secondary coil wound around one or more sectors or sections of the magnetic core. Solid toroidal magnetic cores generally provide the best electrical performance for current transformers, i.e., outputting small current or voltage signals in direct proportion to, and in phase with, the current flowing in the high power primary wire or bus bar with minimal errors, and other solid (not split) core configurations, for example, square or rectangular loops are also quite good. For simplicity and convenience, the term “solid core” or adjective “solid-core” in this description includes any such toroidal, oval, square, rectangular, or other shaped solid (not split) magnetic core. However, to install a current transformer with a solid core onto a high power wire or bus bar, the high power or bus bar has to be inserted through the center hole or aperture of the solid core, which requires disconnecting the high power wire or bus bar from its high power circuit and inserting it through the solid core, and then reconnecting the high power wire or bus bar to the high power circuit.
Current transformers equipped with split magnetic cores, often called “split-core” current transformers, alleviate this inconvenience by enabling the core to be opened or disassembled for installation around a high power wire or bus bar and then closed or reassembled for operation without having to disconnect the high power wire or bus bar from its circuit. A typical split magnetic core may comprise two semicircular halves of a toroidal magnetic core, two C-shaped halves or other portions of a square or rectangular magnetic core, two U-shaped halves or other portions of an oval magnetic core, a U-shape magnetic core section with a closing-bar core section extending from one leg of the U-shape section to the other leg, and other core section configurations that can be opened or disassembled. However, a magnetic core that is split, so that it can be opened or disassembled, has unavoidable air gaps in the magnetic core, thus increasing the magnetic reluctance, which in turn decreases the permeability and causes higher excitation current, all of which increases the secondary coil output errors, particularly the phase angle error between the phase of the current in the high power wire or bus and the phase of the output current or voltage from the secondary winding. Consequently, while split-core current transformers are generally more convenient and easier to use than solid-core current transformers for many installations and circumstances, the electrical performance of split-core current transformers is not as good as comparable sized and shaped solid-core current transformers, assuming all other factors are constant, and typical split-core current transformers also draw more magnetizing current than solid-core transformers made with the same core material and of the same size. Also, while split-core current transformers alleviate the need to disconnect the high power wire or bus bar for installation, as explained above, they need bracketry and mechanisms to clamp or hold the spit-core components together upon installation on a high power wire or bus bar, which is more complicated than solid-core current transformers and can be somewhat cumbersome to use.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.